While a vehicle is traveling in congested traffic it may be desirable to stop the vehicle's engine to conserve fuel. However, stopping an engine can cause air to be pumped through a catalyst positioned downstream of the engine. The air in the catalyst may allow higher levels of NOx to be released from the vehicle's exhaust system. On the other hand, it may be desirable to pump some oxygen into the catalyst so that oxygen is available to oxidize hydrocarbons when the engine is restarted. Thus, there may be conflicting requirements as to whether or not it is desirable to pump air through the engine during engine stopping.
The inventor herein has recognized the above-mentioned disadvantages associated with frequent automatic engine stopping and starting and has developed a method for operating an engine, comprising: shutting down an engine; and adjusting current supplied to an electric device applying torque to a crankshaft of the engine in response to an oxygen storage capacity of a catalyst at a time of shutting down the engine.
By adjusting current supplied to an electric device applying torque to a crankshaft of an engine, it may be possible to better control an amount of air that is pumped into a catalyst when an engine is stopped. For example, if the catalyst has a high oxygen storage capacity and a low amount of oxygen stored in the catalyst at a time when an engine stop is requested, the engine may be allowed to rotate a predetermined first number of times from initiation of the engine stop to the time engine speed is zero. Alternatively, if the catalyst has a high oxygen storage capacity and a large portion of the available oxygen storage capacity is utilized at the time of an engine stop request, the engine may be allowed to rotate a predetermined second number of times from initiation of the engine stop request to the time engine speed is zero. In one example, the second number is smaller than the first number so that less air may be pumped through the catalyst by the engine when a large portion of the catalyst's oxygen storage capacity is utilized. In this way, engine stopping can be controlled to adjust the operating state of the catalyst in preparation for an engine restart.
The present description may provide several advantages. Specifically, the approach may reduce engine emissions during engine starting. Additionally, the approach may be applicable to a variety of electrical machines that work with the engine. For example, the approach may be implemented with a starter that is engaged via a pinion. Further, the approach may be implemented with an integrated starter/alternator that is coupled to the engine's crankshaft via a belt. Further still, the approach may be applicable to a system where an electric machine is mechanically coupled directly to the engine crankshaft.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.